Generally, active-matrix display apparatus display images by arranging a large number of pixels in a matrix and controlling the light intensity of each pixel according to a video signal. For example, if liquid crystals are used as an electrochemical substance, the transmittance of each pixel changes according to a voltage written into the pixel. Even with active-matrix display apparatus which employ an organic electroluminescent (EL) material as an electrochemical substance, the basic operation is the same as in the case of using liquid crystals.
In a liquid crystal display panel, each pixel works as a shutter, and images are displayed as a backlight is blocked off and revealed by the pixels or shutters. An organic EL display panel is of a self-luminous type in which each pixel has a light-emitting element. Consequently, the self-luminous type display panel such as an organic EL display panel has the advantages of being more viewable than liquid crystal display panels, requiring no backlighting, having high response speed, etc.
Brightness of each light-emitting element (pixel) in an organic EL display panel is controlled by an amount of current. That is, organic EL display panels differ greatly from liquid crystal display panels in that light-emitting elements are driven or controlled by current.
A construction of organic EL display panels can be either a simple-matrix type or active-matrix type. It is difficult to implement a large high-resolution display panel of the former type although the former type is simple in structure and inexpensive. The latter type allows a large high-resolution display panel to be implemented, but involves a problem that it is a technically difficult control method and is relatively expensive. Currently, active-matrix type display panels are developed intensively. In the active-matrix type display panel, current flowing through the light-emitting elements provided in each pixel is controlled by thin-film transistors (transistors) installed in the pixels.
Such an organic EL display panel of an active-matrix type is disclosed in Japanese Patent Laid-Open No. 8-234683. An equivalent circuit for one pixel of the display panel is shown in FIG. 62. A pixel 16 consists of an EL element 15 which is a light-emitting element, a first transistor 11a, a second transistor 11b, and a storage capacitance 19. The light-emitting element 15 is an organic electroluminescent (EL) element. According to the present invention, the transistor 11a which supplies (controls) current to the EL element 15 is referred to as a driver transistor 11. A transistor, such as the transistor 11b shown in FIG. 62, which operates as a switch is referred to as a switching transistor 11.
The organic EL element 15, in many cases, may be referred to as an OLED (organic light-emitting diode) because of its rectification. In FIG. 62 or the like, a diode symbol is used for the light-emitting element OLED 15.
Incidentally, the light-emitting element 15 according to the present invention is not limited to an OLED. It may be of any type as long as its brightness is controlled by the amount of current flowing through the element 15. Examples include an inorganic EL element, a white light-emitting diode consisting of a semiconductor, a typical light-emitting diode, and a light-emitting transistor. Rectification is not necessarily required of the light-emitting element 15. Bidirectional diodes are also available. While the reference numeral 15 is described as an EL element, it is sometimes used as the meaning of an EL film or an EL structure.
In the example of FIG. 62, a source terminal (S) of the P-channel transistor 11a is designated as Vdd (power supply potential) and a cathode of the EL element 15 is connected to ground potential (Vk). On the other hand, an anode is connected to a drain terminal (D) of the transistor 11a. Besides, a gate terminal of the P-channel transistor 11b is connected to a gate signal line 17a, a source terminal is connected to a source signal line 18, and a drain terminal is connected to the storage capacitance 19 and a gate terminal (G) of the P-channel transistor 11a. 
Incidentally, although it is stated herein that the transistor elements 11a which supply current used to drive the EL elements 15 are p-channel transistors, this is not restrictive and they may be n-channel transistors. Of course, the transistors 11 may be bipolar transistors, FETs, or MOSFETs. The board 71 is not limited to a glass substrate and may be a silicon substrate or metal substrate.
To drive the pixel 16, a video signal which represents brightness information is first applied to the source signal line 18 with the gate signal line 17a selected. Then, the transistor 11a conducts, the storage capacitance 19 is charged or discharged, and gate potential of the transistor 11b matches the potential of the video signal. When the gate signal line 17a is deselected, the transistor 11a is turned off and the transistor 11b is cut off electrically from the source signal line 18. The gate potential of the transistor 11a is maintained stably by the storage capacitance 19. Current delivered to the light-emitting element 15 via the transistor 11a depends on gate-source voltage Vgs of the transistor 11a and the light-emitting element 15 continues to emit light at an intensity which corresponds to the amount of current supplied via the transistor 11a. 
Organic EL display panels are made of low-temperature polysilicon transistor arrays. However, since organic EL elements use current to emit light, there has been a problem that variations in the characteristics of the transistors will cause display irregularities.